1. Field of the Invention
The present invention relates to a tuning system for use in transmitters and receivers in radio, television, stereo tuners, and personal radio communications, and other communication equipment.
2. Description of the Prior Art
As more and more waves are used in radio and television broadcasting and communications, tuning systems for selecting the frequencies of waves to be received are required to be highly stable and reliable in their performance. There has also been a great demand for the reduction of the cost of manufacture of receivers, transmitters and other communication equipment in which the tuning systems are installed. In particular, the development of a new technology is desired for tuning system circuit components in radio-frequency stages which have been difficult to improve.
Conventional tuning systems will be described with reference to the drawings. FIG. 1 of the accompanying drawings shows a basic circuit arrangement of a tuning oscillator device. The tuning oscillator comprises a tuning circuit 4 composed of a tuning inductor 1, a variable capacitor 2, and a fixed capacitor 3. To the tuning circuit 4, there is connected an input terminal or output terminal 6 of a feedback amplifier 5.
FIG. 2 illustrates another circuit arrangement of a conventional tuning amplifier device, which comprises a tuning circuit 10 composed of a tuning inductor 7, a variable capacitor 8, and a fixed capacitor 9. A signal applied to an input terminal 11 is amplified by an amplifier 12 which supplies an amplified output through an output terminal 13 to the tuning circuit 10.
FIG. 3 shows a circuit arrangement of a conventional mixer device including a tuning circuit 16 comprising a tuning inductor 14 and a fixed capacitor 15. A mixer 19 has a first input terminal 17, a second input terminal 18, and an intermediate-frequency output terminal 20 connected to the tuning circuit 16.
FIG. 4 shows conventional components of which the tuning circuits 4, 10, 16 shown in FIGS. 1 through 3 are composed. The components include a tuning inductor 21, a variable capacitor 22, and a fixed capacitor 23 which are interconnected by circuit conductors 24, 25.
However, the conventional arrangements have been subjected to the following disadvantages:
In the tuning system shown in FIG. 4, the inductor component 21 is large in size as compared with the other components, and is particularly much larger in height, with the result that the equipment in which the tuning system is incorporated is not rendered small in size and low in profile. A ferrite core inserted in the inductor component is variable in position due to mechanical vibrations, resulting in wide variations in tuning frequencies. The inductance of the inductor component is unstable due to a large degree of temperature-dependency of the magnetic permeability of the ferrite core, a feature which also causes tuning frequencies and also Q of the resonator filter circuit to vary widely. For keeping tuning frequencies stably at their target settings, the components are required to be installed highly accurately in predetermined positions. Where the tuning systems are mass-produced as RF resonator filters, it is difficult to maintain a desired installation accuracy and hence the tuning frequencies tend to differ largely from their target settings and cannot be caused to converge to fixed values. Therefore, there has been difficulty experienced with the mass production of the tuning systems.
The tuning system arrangement shown in FIG. 4 has other drawbacks. The inductor and the capacitor are constructed as separate components that are interconnected by long conductors. The long conductors tend to produce unwanted lead inductances and stray capacitances which cause the tuning circuit to operate unstably and make it difficult to achieve an initial design target. Accordingly, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no tuning systems of desired design can be achieved. This invites abnormal oscillation, oscillation of unwanted signals, increased higher harmonics in oscillated signals, resulting increased distortion, and a narrowed width in which a variable tuning oscillation frequency varies. Furthermore, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no tuning amplifiers of desired design can be achieved. This invites abnormal oscillation, response of unwanted signals, increased higher harmonics in amplified signals, resulting increased distortion, a narrowed width in which a variable tuning oscillation frequency varies, and deterioration in abilities to remove intermodulation interference and spurious interference. Also, no sufficient frequency selectivity can be ensured, and undesired resonant conditions are produced at uncertain frequencies, with the consequence that no intermediate-frequency tuning circuits of desired design can be achieved. This invites abnormal oscillation, generation of spurious signals, increased distortion caused by increased higher harmonics in intermediate-frequency signals, and deterioration in abilities to remove intermodulation interference and spurious interference.
Since the tuning circuits are composed of discrete components having minimum unit functions, there is a limitation on efforts to reduce the number of parts used and to improve the manufacturing process.